Liquid-crystal displays, plasma displays, active matrix organic EL (active matrix organic light-emitting diode (AMOLED)) displays, and the like are conventionally used in the technical field of flat panel displays. In terms of the light source of these displays, liquid-crystal displays are classified as back-lit displays, whereas plasma displays and active matrix organic EL displays are classified as self-emitting displays.
Back-lit liquid-crystal displays are known to have poor bright-dark contrast compared to self-emitting displays because it is necessary for the backlight thereof to be switched on even when displaying a dark image and thus the screen luminance is not sufficiently reduced (referred to as “black floating”, or “black level maladjustment”).
A technique of divided control of switched-on regions of a backlight has been adopted to solve this problem of “black floating”. This technique is referred to as partial driving, and may also be referred to as “area driving”, “backlight driving”, or “local dimming”. In a partial drive-type backlight, LEDs that serve as a light source of the backlight are switched on brightly at sections within a single screen where a bright image is to be displayed (bright sections). On the other hand, the luminance of LEDs is reduced at sections where a dark image is to be displayed (dark sections). This improves the luminance contrast between bright sections and dark sections of an image.
However, even in the case of a partial drive-type backlight, it may not be possible to benefit from the improvement in luminance contrast provided by partial driving in a situation in which the number of screen divisions for which partial control is performed is small and in which both a bright section and a dark section are present within a single partially driven control region. This is because when both a bright section and a dark section are present within a single control region, it is necessary for LEDs to be switched on with high luminance in accordance with this bright section, and thus light from the backlight faintly leaks into the partial dark section within the control region.
Therefore, it is preferable that the number of partial drive divisions of a screen is a large as possible such that the partial driving functions effectively at a fine level with respect to the screen. For this reason, partial drive-type backlights referred to as “direct backlights” that enable a large number of divisions to be made are attracting attention.
A light source device that functions as a backlight is typically formed from a light-emitting section that includes green and red light-emitting phosphors or a yellow light emitting phosphor, and LEDs that serve as an excitation light source for the phosphor(s). A phosphor is commonly used in one of the following three configurations. In a first configuration, the phosphor is mixed with a resin material and the resultant mixture is used to cover an LED chip. In a second configuration, the phosphor is applied directly onto a light emission surface of an LED. In a third configuration, a structure in which an LED and a phosphor sheet that contains the phosphor are at separated positions (hereinafter, referred to in this specification as a “remote phosphor system”) is adopted (for example, PTL 1).
The following provides a more specific description of a remote phosphor system direct backlight (i.e., a backlight having the third configuration described above) according to conventional techniques with reference to the schematic view illustrated in FIG. 1. In the direct backlight illustrated in FIG. 1, blue LEDs 10B that serve as an excitation light source are arrayed in a grid shape on a chassis 51 and a reflection sheet 52 is disposed at the inside of the chassis 51, except for at positions where the blue LEDs 10B are disposed. A diffusing plate 40, a phosphor sheet 20, and an optical sheet group 70 are disposed in this order above the blue LEDs 10B and are held by a sheet securing member 53. Note that the phosphor sheet 20 is separated from the blue LEDs 10B. When the blue LEDs 10B serving as an excitation light source for the phosphor sheet 20 emit light, this light is incident on the phosphor sheet 20 as excitation light, and the phosphor sheet 20 releases emitted light in a wavelength region from red to green. The incident light from the blue LEDs 10B and the emitted light from the phosphor sheet 20 combine to form white light.
At present, the first configuration is most commonly adopted among the first to third configurations described above. However, in the first and second configurations, the phosphor may be directly affected by heat and light resulting from LED light emission. Moreover, in the case of a phosphor that is easily degraded by moisture in air, it may be necessary to adopted a structure in which the whole of the resin in which the phosphor is dispersed in protected from moisture. Accordingly, there are many cases in which the third configuration is preferable to the first and second configurations for technical and cost-related reasons. Therefore, the use of a remote phosphor system corresponding to this third configuration is attracting interest for cases in which a phosphor that is easily degraded by heat or light or a phosphor that is easily degraded by moisture is used.